1. Field of the Invention
The field of the invention relates to printing presses including impression cylinders having one or more gripper assemblies for gripping the leading edges of successive sheets of paper.
2. Brief Description of the Related Art
Multi-color printing presses have been employed for printing sheets of paper sequentially at a plurality of printing stations with inks of different colors. Such presses may include an impression cylinder, an entrance cylinder for providing sheets of paper to the impression cylinder, an exit or delivery cylinder for removing printed sheets of paper from the impression cylinder, and a plurality of printing stations located between the entrance and exit cylinders. Each printing station typically includes a blanket cylinder engageable with the impression cylinder. Four such printing stations are frequently employed. The impression cylinder is often referred to as an impression drum in this type of application.
It is of paramount importance that sheets of paper fed to the impression cylinder are gripped so as to maintain proper registration as they are transported through the various printing stations. Small errors resulting from misregistration are readily observable in the printed sheets, and accordingly must be avoided.
The impression cylinder of a multi-color printing press usually supports a number of sheets of paper, each of which is gripped at its leading edge. A number of gripper assemblies are accordingly mounted within the impression cylinder, one for gripping the leading edge of each sheet supported by the cylinder. Each gripper assembly includes a plurality of grippers, also known as fingers, which are mounted to a shaft. The shaft is operated by a cam which opens and closes the grippers at the proper times to grip and release a sheet of paper. The fingers are urged towards the closed position by a spring. A spring may also be provided for absorbing the shock of the closing of the grippers. U.S. Pat. No. 4,375,190, which is incorporated by reference herein, discloses a four color printing press having eight gripper assemblies as described above.
Various other gripper assemblies have been designed for gripping the leading edges of sheets of paper within a printing press. U.S. Pat. Nos. 3,926,118, 4,014,261, 4,448,125, 4,475,459 and 4,815,379 disclose examples of such gripper assemblies.
As discussed above, it is important to grip a sheet of paper with sufficient force such that proper registration is maintained as it passes through the printing stations. The gripping force should not be excessive, however, to avoid leaving impressions on the sheets of paper.
The printing stations are often required to print sheets of paper near the leading edges thereof, including edge areas normally engaged by a gripper. The grippers should accordingly be individually removable to allow such printing.